This invention relates to apparatus for continuous mixing. More particularly, this invention relates to apparatus for continuous mixing where the material being mixed is unstable such as explosives and the like.
Apparatus for continuously mixing, blending and/or extruding have been developed over the years and utilized in many continuous processing systems. Such apparatus have been used in such diverse applications as polymer manufacture, compounding and finishing systems, rubber mixing, grafting and degasing, food processing and hazardous waste solidification. Other applications have included fossil fuel pelletizing and agglomeration, and the continuous production of adhesives, powder coatings, catalysts and the like.
A particular challenge to those concerned with the design and manufacture of continuous mixing equipment is the challenge presented by the mixing of unstable materials such as explosives. For example, the manufacture of plastic-bonded explosives (PBX) involves the mixing of a liquid polyester matrix with explosive material in pellet or granular form to provide a homogeneously mixed product. There must be a consistent dispersion of the explosive material in the matrix and no reduction in the explosive grain size. In achieving an acceptable product, care must be taken not to cause shear stresses in the explosive material beyond defined limits because exceeding such limits may well result in detonation of the explosive material with catastrophic results.
Known continuous mixing equipment is not particularly well suited for these applications. They operate based upon pressure generation, mixing and kneading to create desired shear and mixing intensities. There are structured relationships in the equipment such as at hinges and bearings where high shear forces may be exerted in the product being processed, which forces are unacceptable in handling explosives and other unstable materials. Very specific problems include the inability to quickly open the process section so as to avoid pressure build-up or excessively high temperatures, as well as difficulty in cleaning the equipment, particularly the bearings and hinges.
In most known continuous mixers, the bearings which support the mixing screws for rotation are mounted on or integral with the barrel structure. As a result, product being mixed often gets into the moving bearing elements. Although this may not be a problem for mixing some materials, this is a very definite problem for materials such as explosives because any explosive material which enters the bearing and becomes subject to the moving parts may experience shear stresses which exceed the explosive limit with predictable and catastrophic results.
With regard to cleaning, some mixers are disassembled for cleaning by removing the screws from the barrel. Others have axially split barrels with the sections being hinged together so as to rotatably swing away from the screws. Again, while acceptable for some materials, such structures are not desirable for materials such as explosives. Removal of the screws from the barrel often is accompanied by the screws accidentally striking the barrel surface. Where the mixer has been used to mix explosives there is a residuum of explosive material which may well be detonated by the striking of the screws against the barrel. As to hinged barrel sections, they are subject to having explosive material find its way into the hinge mechanisms which, in response to rotation of the hinges, may detonate.
Another problem with other known mixers is that the flow-path of the material through the apparatus is such that dead spots are formed, i.e., spots where there is no continuous movement thus increasing the residence time of the material. Increased residence time is unacceptable as it increases the amount of energy added to the material which may lead to degradation and detonation.
For the foregoing reasons, most mixing of explosive and otherwise unstable materials occurs by batch mixing. The products are mixed in batches ranging from five to ten thousand pounds per batch. This, of course, is a long and tedious process and far inferior to continuous mixing.